Computing systems can provide information that has been derived from a variety of different sources. For instance, some computing systems provide information that has been derived from one or more sensors that measure aspects of the physical world. A computing device, for example, that is equipped with a GPS sensor can provide information about the location of the computing device. Information, however, may decrease in certainty over time. For example, if a GPS sensor determines the location of a computing device, the information related to the location of the computing device may be less certain one hour later unless updated in the interim. Accordingly, computing systems would like to be able to represent the certainty of information as time passes and the information decreases in certainty.